To be honest just a simple red point at the correct location is all you need (see attached screenshot showing idea for buttons for horizontal lines), using software like CAD it's all about the visual experience for most 'end users' - so I think users would soon get the idea and of course you would have the tool tips when hovering over the buttons with the mouse.

It's always difficult to decide what should be represented as icon and what as text. I also get feedback from users who think that all tool buttons should only contain texts (e.g. 'lin', '2P', etc.)! Of course arguably, one could also make everything customisable, but that has the downside that there would be no 'one' QCAD anymore as QCAD would look totally different on each installation. Anyway, let's see if this issue gets any votes / other opinions.

The option you have added in 3.7.6 for 'Draw Rectangle with size' - 'Add hotkeys to change reference point [1-9]' would be ideal to add this option for aklso selecting reference points for tools like the Horizontal and Vertical Line tools (and others).